There is a need in modern manufacturing processes to measure the lateral displacement of two objects, or of features on the objects, automatically and with great precision where the objects are in separate planes and can't be conveniently placed in the same or adjacent planes. For example, in integrated circuit manufacture, it is necessary to align the patterns on a projection exposure mask with existing patterns on a radiation sensitive resist coated semiconductor wafer so that correct overlay of the patterns will occur when the resist is exposed. Also, it is necessary to check sets of exposure masks prior to use to assure that the patterns on different masks overlay one another within acceptable tolerances.
The integrated circuit device density is constantly being increased to permit more circuits to be formed in a given area of semiconductor. This requires that the positioning and measurement be accurate to smaller and smaller dimensions. In the case of mask and wafer alignment for projection printing, it is customary to provide corresponding alignment targets on the mask and wafer. In the past, the targets have been manually superimposed by an operator. This is a slow process and the alignments suffer from operator fatigue and error. Electro optical alignment systems have been devised to provide automatic alignment but these have not replaced operator alignment. One reason is the fact that, because of target quality and the number of layers of material which may overlay a target, the alignment signals do not permit successful alignment a sufficient percent of the time to achieve the degree of reliability needed for a production line.
U.S. Pat. No. 3,957,376 discloses a measuring method and system for measuring the dimensional characteristics of line edges or boundaries of patterns on a semiconductor wafer using a diffraction pattern which is produced by two parallel, physical edges which are located in the same or substantially the same plane.
Where two objects are involved, it may not be possible to locate the edges in the same plane. U.S. Pat. Nos. 3,797,939; 3,883,249 and 3,884,581 disclose the determination of boundary positions by interference pattern analysis where the boundaries can be in different planes along the path of the light and in which the interference waves are produced by the physical boundaries. In the case of mask and wafer alignment one of the boundaries is located on an opaque surface so that a useful interference pattern cannot be produced by this means.
We have now found a system and method which provides for measuring the lateral displacement of edges located in different planes to a precision in the order of 1 microinch using a diffraction pattern produced by the physical edge in one plane and the aerial image of the other edge located in the plane of the physical edge. Surprisingly, this diffraction pattern provides a precision comparable to that produced by two physical edges in the same plane. The signal to noise ratio is high and provides useful signals where usually low contrast alignment targets are involved and/or where the alignment targets are covered by overlying layers.